Well logging method



Sept. 7, 1943. H. HOOVER, JR 2,323,555

v WELL LOGGING METHOD Filed May 27, 1940 8 I 1 12 5-3- 9 Hyena canoe/vCGNtt/VTEA 770A! {8) Fz'ml- A ZL ::x A. 5.. 2 1 6 l6 v D... E Q a 3 1234 .J? Ti4:F1EZ

f1" 5-4 HmRocAeawv Can/cumin .Ds TH INVEN TOR. HERBERT Ham/ER J2Patented Sept. 7, 1943 WELL LOGGING METHOD A Herbert Hoover, Jr., SierraMadre, Calit, assignor to Consolidated Engineering Corporation,Pasadena, Calif., a corporation of California Application May 27, 1940,Serial No. 337,503 2 Claims. (mess-1.4).

This invention relates to a method for logging the formations of an oilwell during the course of drilling and in particular to a well loggingmethod in which measurements are made of the concentration ofhydrocarbons occurring in the formations encountered as drillingprogresses.

According to my invention I analyze drilling mud for hydrocarbon contentduring the course of drilling, and in this way determine the amount ofhydrocarbons present in the drilling mud as a result of contact withsubterranean strata. In order to obtain a hydrocarbon Well log which isaccurate, I obtain samples of drilling mud prior to passage down thedrill pipe to the point at which the drilling is taking place and laterobtain a corresponding sample of drilling mud that returns to thesurface after making a single complete cycle. By analyzing the twosamples, which will hereinafter be referred to respectively as the inand out samples, for their respective hydrocarbon con-tents, I am ableto measure the net change in hydrocarbons produced in the drilling mudas a result of contact with the subsurface.

Occasionally the net change of drilling mud hydrocarbon concentration isa decrease caused for instance from an influx of connate waters into thedrilling mud. However, when operating in accordance with the preferredform of my invention the net change is an increase representing both thehydrocarbons which have dissolved in the drilling mud and those retainedin the drill cuttings.

The measured quantities of hydrocarbons may be representative ofhydrocarbons which have migrated upward from a deep deposit underlyingthe stratum from which the cuttings originate, or they may signify aconcentrated petroleum deposit which may have commercial value. Thedistinction between the two possible interpretations to be given to thedata obtained, depends partly on whether an increase in hydrocarbonconcentration with depth is observed and I partly on the total quantityof hydrocarbon observed.

As is well known, it is often necessary to make an electrical survey ofa well subsequent to the completion of drilling in order to ascertainwhether an oil sand has been overlooked. In case large quantities ofhydrocarbons of possible commercial importance exist in the stratumbeing penetrated by the drill,'the existence of such large quantitiesmay be detected according to my invention at the surface during thecourse of drilling without any danger of being overlooked because ofbeing sealed off by the mud cake accumulating on the bore hole wall.

Small quantities of hydrocarbons returned with the drilling mud,increasing in concentration with depth indicate the approach of thedrill to a productive horizon. In case the hydrocarbons from the drillmud are hydrocarbons which have migrated from deep lying .deposits, theexistence of such deposits may therefore be detected prior to thepenetration thereof by the drill.

My invention is also useful in detecting the nature of the depositsexisting in the strata penetrated by the drill. In this way I candetermine whether a petroleum deposit encountered has the gravity orother characteristic suitable for commercial purposes.

By determining the relative proporitons of heavy and light hydrocarbonsreturned with the drilling mud during the course of drilling I am ableto locate the surface of separation between the gas and oil bearinglevels in a petroleum bearing stratum. Thus, I can also locate thegas-oil interface in a productive zone and aid 8. driller in settingcasing at the optimum position in the well prior to a production test.

Accordingly, the principal object of my invention is to provide a methodof exploring for valuable subterranean mineral deposits.

Another object is to provide a method for logging a well during thecourse of drilling which will detect directly hydrocarbons present inthe formations encountered by the drill.

Another object is to provide for obtaining a hydrocarbon well log freefrom any error arising from contaminating hydrocarbons present in thedrilling mud prior to pumping said mud into the well.

Another object is to provide for determining the constituency of apetroleum deposit encountered in the course of drilling.

Another object is to provide a method of detecting the presence of aproducing stratum prior to its penetration by a drill.

Another object is to provide a method for detecting the gas-oilinterface in a productive zone.

My invention possesses numerous other objects and features of advantage,some of which, together with the foregoing, will be set forth in thefollowing description of specific apparatus embodying and utilizing mynovel method. It is therefore to be understood that my method isapplicable to other apparatus, and that I do not limit myself, in anyway, to the apparatus of the present application, as I may adopt variousother apparatus embodiments, utilizing the method, within the scope ofthe appended claims.

In the drawing:

Fig. 1, which is partly schematic and partly in section, shows a typicalorganization of surface drilling equipment to which my invention isapplicable.

Fig. 2 drawn to a larger scale than Fig. 1 shows a section of the earthnear the bottom of a drill stem.

Figs. 3 and 4 represent well logs obtained in accordance with myinvention.

Referring to Figs. 1 and 2, a platform 2 mounted on the surface 4 of theearth has constructed thereon a. derrick 6 which is used to support andmanipulate a drill pipe 8 having a bit In on the bottom end thereof fordrilling into subsurface formations. The drill pipe is connected at itsuppermost end to a swivel l2 which in turn is supported by a travellingblock l4.

- The drill pipe may be raised or lowered vertically by taking up orletting out the drill cable l6 which passes several times over thepulley [8,

'on crown block 19 and a pulley on a travelling block M. The pressure ofthe drill on the bottom 01 the drill hole is controlled by manipulationof the tension in cable l6.

Rotary motion is supplied to the drill pipe 8 and bit III by suitabletransmission of energy from a motor 20 to a rotary table 22 mounted onplatform 2, said rotary table being adapted to transmit motion to thedrill pipe 8 through a kelly 26 which forms the uppermost section ofsaid drill pipe 8.

In order to lubricate the drill bit It! during drilling and to removedrill cuttings from the drill hole, mud of predetermined constituency ispumped from the sump 30 by a pump 32 through the hose 34, and swivel 12into a continuous passage which extends throughout the entire length ofthe drill pipe from kelly 26 to bit [0. When the mud reaches the drillbit it passes into the drill hole and picks up any drill cuttings whichmay be present. The mud laden with cuttings is returned to the surfacethrough the space between the wall 36 of the drill hole and the drill'pipe.

It is clear that in general some of the hydrocarbons which may bepresent in the formation through which the drill is passing, dissolve inthe mud and some are retained by the cuttings.

When the mud returns to the surface it is discharged into the sumpthrough a discharge pipe 38. Due to the fact that any hydrocarbonspicked up by the mud in the course of drilling thus enter the sump andincrease the average hydrocarbon content'of the mud entering the drillpipe and the fact that light hydrocarbon constituents present in thesump continually evaporate, the hydrocarbon contentof mud entering thedrill pipe is in a continually changing state.

According to my invention I obtain an accurate measure of hydrocarboncontent of the formations encountered by the drill during the progtimeby collecting a portion or the drilling mud from the mud faucet 39 orfrom the end of bleeder line 40 in a pan or jar held beneath said faucet39 or the end 44 of bleeder line 40.

The in sample is preferably taken While thedrilling equipment isoperating in a steady, stable condition. In taking an in sample, any mudstanding between themud line and the point ress of drilling by comparingthe hydrocarbon;

of collection is first drawn off so that the in" sample isrepresentative of the drilling mud passing into the drill pipe at thetime said "in sample is collected.

Another, but less satisfactory way, of obtaining an in sample, is .tocollect 'mud directly from the bottom of the kelly 26 or from the hose34 at the time that an additional section of drill pipe is introduced toincrease the drill pipe length for drilling deeper.

I obtain an "out sample directly by collection of a portion of thedrilling mud at the outlet 46 of the discharge line 38. Said out sampleis preferably obtained at a point a close as possible to the drillcollar 49 before any substantial amount of hydrocarbons have had achance to be lost by evaporation. In case there is any considerablepressure on the return drill mud, I prefer to collect and maintain thesample at said pressure. In collecting the out sample I 'include bothmudand cuttings contained therein in order to'get a sample trulyrepresentative of the formation being drilled.

The mud cycle may be measured, for example, by introducing a marker,such as a handful of oats into the mud, just prior to pumping themudinto the drill pipe. The cats may be introduced into the drill pipeif desired when a new stand of drill pipe is added. The oats may laterbe detected in the return mud by any convenient method such as bypassing the return mud through a screen 48 prior to return of the mud tothe sump. A shaker screen which may be used for this purpose is usuallyavailable at the well.

The length of time required for the oats to pass through the drill pipeand return to the surface-is known as the mud cycle.

To obtain a log 'of the formations encountered by the drill, I correlatethe in and out samples together with the corresponding depth in a singlemud cycle.

From the pumping rate and the-length and cross-section of the drill pipeand drill hole, I can determine the position of any particular portionof mud in the drillsystem at any time after taking an in sample.Accordingly, I can compute the length of time required for mud to passfrom the "pump 32 down to the formation being drilled. Thus, I mayreadily determine the time and the corresponding depth at which the massof mud corresponding to any given in sample will pick up cuttings at thebottom of the drill hole and when this mud sample will return to thesurface with said drill cuttings and any hydrocarbons contained in theportion of formation being drilled away. v

In the preferred form of my invention, the formations previously passedthrough by the drill such' as strata 50, 5|, and 52 above the drillingpoint cannot make any substantial contribution to the hydrocarboncontent of the returning drill mud, first by virtue of the fact that amud cake is usually formed on the wall 36 of the drill hole effectivelysealing off the formations and secondly. by virtue of the fact thatI'maintain the densityof the drilling mudhigh enough for the 2,828,555hydrostatic head of the drilling mud to exceed any formation pressurewhich I expect to encounter. Thus, the increase in hydrocarbon contentof any portion of mud under examination is representative of theformation being penetrated by the drill. In order to measure theconcentration of hydrocarbons in the formation being drilled I comparethe increase in hydrocarbon content of the mud with the amount offormation drilled through during the passage of a standard quantity ofmud.

Since the volume of subsurface stratum penetrated during the course ofdrilling is usually very small compared with the amount of mud passingthe drilling point, the concentration of hydrocarbons per unit volume offormation may usually be expressed by the equation M oi)fi whereHO=hydrocarbon concentration in out sample H1 =hydrocarbon concentrationof in sample M =mud rate, 1. e., volume of mud flowing past any point inthe drillin system per unit time D =drilling rate, i. e., volume offormation being removed by the drill per unit time.

. trations Ho and Hi, corresponding respectively, to

the out' and in drilling mud samples, I cc'llect these samples in anyconvenient manner such as that hereinabove described, place the samplesin airtight containers, make records of the depths to which said samplescorrespond, record the mud and drilling rates and transport said samplesto a central laboratory where the hydrocarbon concentrations of thesamples may be measured by any convenient method either chemical orphysical. 1

I prefer to use a mass spectrometer for my analysis of the in" and "outsamples, since with this instrument I may obtain a complete analysis ofthe various hydrocarbon constituents of the drilling mud and therebprovide data for determining the nature of the hydrocarbons which arecontained in the strata under investigation. By making a completeanalysis I am thus able to determine whether any petroleum depositencountered possesses commercially desirable qualities.

In Fig. 3 I have shown a typical graph of total formation hydrocarbonconcentrations (S) obtained by my method. Down to the depth A it is seenthat only small concentrations of hydrocarbons gradually increasing withdepth are present, but beginning from the depth A to the depth B thehydrocarbon concentration rapidly increases indicating the approach ofthe drill to a productive horizon, while at depth C the total quantityof hydrocarbons found indicates that a saturated oil sand exists at thislevel. Similarly, the gradual increase of hydrocarbon concentration, inthe region of point D indicates the approach of an oil sand located atthe depth E.

,If desired, separate logs may be made of individual hydrocarbonconstituents detected. In Fig. 4 I have shown on an enlarged depth scalea plot of the concentrations of both heavy and light hydrocarbons foundin a productive zone. For my purpose hydrocarbons heavier than butaneare considered heavy while butane and lighter hydrocarbons may beconsidered light. The division into light and heavy is more or lessarbitrary and may be varied according to the rev quirements of the area.For the purpose of loeating a gas-oil interface, samples are preferablytaken at closely spaced points, preferably less than two feet apart. Byan examination of a heavy-light hydrocarbon log adriller is enabled toset casing at such a point that a good .pro-

duction rate may be obtained without excessive loss of formation energy.The bottom of the casing is preferably set a short distance below theleast depth R where the heavy hydrocarbon concentration reaches a highvalue. A water-oil interface may also be detected in accordance with mymethod.

It frequently happens that certain strata which persist throughout alarge area contain characteristic concentrations of hydrocarbons whichare of no commercial value. While m invention is primarily useful indetecting a productive horizon either directly or by the approach of adrill to a producti e horizon, it may also be used to correlate thedepths at which such characteristic marker beds appear in differentholes drilled in an area under investigation.

In case an abnormally large concentration of any particular hydrocarbonis found in any stratum in excess of the concentration characteristic ofsuch stratum, it is usually indicative of a substantial deposit ofpetroleum within or near such stratum.

In general my invention provides a method of hydrocarbon well-logging inwhich correction is made for the previous history of the drill mud, bycomparing the hydrocarbon concentrations of two mud samples, one takenat the beginning and one at the end of a common mud circulation cycle.The concentration of hydrocarbons in the formation being penetrated bythe drill is computed by taking into account the rate at which the drillis penetrating the formation under test and the rate at which mud iscirculating through the drilling system.

While I prefer to maintain the mud pressure substantially higher thanthe pressures in any formation encountered in order to avoid substantialdiffusion or flow of hydrocarbons into the drilling mud, it is clearthat my invention will also be applicable even when the mud pressurefalls below the pressure in a formation already penetrated by the drill.The effect of such a lowering in pressure will be merely to produce asubstantially constant hydrocarbon concentration background as a resultof continual influx of hydrocarbons from such a formation into the mud.The magnitude of such a background varies directly as a function of thedifference in mud and formation pressures and also inversely as afunction of the length of the mud cycle.

Many variations may, of course, be made in my method within the spiritof my invention, and the scope of the appended claims. For example, theconcentration of hydrocarbons may'be expressed in terms of the weight ofthe subsurface formation instead of the volume, or the volume ofcuttings returned to the surface may be measured directly if desired andthe significant concentration of hydrocarbons expressed in terms of thevolume of the cuttings returning in a unit length of time instead ofcomputing the volume of formation being penetrated from the size of thedrill hole and the rate of penetration.

It is sometimes desirable to maintain the mud in the sump substantiallyhomogeneous by continual mixing thus making it unnecessary to collect anin sample for every out sample.

This is especially useful when out samples are obtained in rapidsuccession.

While I prefer to analyze the total contents of in and out drilling mud,I also recognize the fact that under many conditions the gases presentin the mud water will reach an equilibrium condition with respect to thegases present in the solid particles suspended in the mud. Accordingly,under such conditions the hydrocarbon content of the drilling mud waterwill be a measure of the total quantity of hydroca bon present in themud.

In one form of my invention I therefore extract water samples at theinlet and outlet ends. of the drilling system and compare theirhydrocarbon contents for detecting petroleum bearing deposits. Suchwater samples may be obtained, for instance, by screening out thecuttings and precipitating or filtering out suspended matter.

It is to be understood that my invention is also applicable .to thedetection of subsurface min-' eral deposits other than petroleum.

I claim:

1. In a well drilling operation, the improvement which compriseslocating a gas-oil contact in a petroleum bearing zone by determiningthe increments of light hydrocarbons and the increments of heavyhydrocarbons in a, series of portions of the mud column circulated pastthe bottom of the well, said portions being substantially undiluted byinfiltration of fluids from said zone but containing cuttingscorresponding to successive bottom points in said zone at known depthsin the well, some of said points being above and some below the gas-oilcontact, and determining the depth in said zone at which the incrementof heavy hydrocarbons increases rapidly as compared with the incrementof the light hydrocarbons.

2. In exploring a'petroleum bearing zone having a gas-oil contacttherein during a well drilling operation in which cuttings are carriedout of the well by a column of mud circulated be'- the respectiveportions of the mud column have v accumulated cuttings from a series ofbottom points in said zone and at known depths, some of said bottompoints being above and some below the gas-oil contact, maintaining thedensity of the mudcolumn so high that the hydrostatic head exerted bythe mud column in the well is in excess of the formation pressureexerted by the fluids in said zone thereby preventing inflow tions ofthe mud column corresponding to the 1 series of bottom points, wherebythe depth of said gas-oil contact may be determined. p

HERBERT HOOVER, JR.

